Connecting Thermocouple Signals to a DAQ Device

Included in the Section

Before You Begin

This document provides step-by-step instructions for wiring and configuring your NI DAQ device for use with a thermocouple. Before you begin using your DAQ hardware, you must install your application development environment and NI-DAQmx driver software. Refer to the Installing LabVIEW and NI-DAQmx document for more information.

Thermocouple Measurement Fundamentals

Thermocouples are the most commonly used temperature sensors. A thermocouple is created when two dissimilar metals touch and produce a small open-circuit voltage that corresponds to temperature. This thermoelectric voltage is known as Seebeck voltage and is nonlinear with respect to temperature.

Thermocouple Types

Thermocouples differ in composition and accurate range:

Thermocouple Type

Positive Conductor

Negative Conductor

Temperature Range (°C) for Polynomial Coefficients or for Table Conversion

Temperature Range (°C) for Inverse Polynomial Coefficients

J

Iron

Constantan

-210 to 1200

-210 to 1200

K

Chromel

Alumel

-270 to 1372

-200 to 1372

N

Nicrosil

Nisil

-270 to 1300

-200 to 1300

R

Platinum-13% Rhodium

Platinum

-50 to 1768

-50 to 1768

S

Platinum-10% Rhodium

Platinum

-50 to 1768

-50 to 1768

T

Copper

Constantan

-270 to 400

-200 to 400

B

Platinum

Rhodium

0 to 1820

250 to 1820

E

Chromel

Constantan

-270 to 1000

-200 to 1000

Table 1. Thermocouple Types

Cold-Junction Compensation

Thermocouples require some form of temperature reference to compensate for unwanted parasitic thermocouples. A parasitic thermocouple is created when you connect a thermocouple to your measurement hardware. Because the terminals on the hardware are made of a different material than the thermocouple wire, voltage is created at the junctions, called cold junctions, which changes the voltage output by the actual thermocouple. You can measure the temperature at this reference junction with a direct-reading temperature sensor, such as a thermistor or an IC sensor, and then subtract the parasitic thermocouple thermoelectric contributions. This process is called cold-junction compensation (CJC). You must specify your CJC source or a constant value (typically 25°C) when you configure your thermocouple measurement in software.

Configuring a Thermocouple Measurement

You can use NI MAX to quickly verify the accuracy of your measurement system Set Up. Using an NI-DAQmx Global Virtual Channel you can configure a thermocouple measurement without any programming. A virtual channel is a concept of the NI-DAQmx driver architecture used to represent a collection of device property settings that can include a name, a physical channel, input terminal connections, the type of measurement or generation, and scaling information.

Follow these steps to begin:

With MAX open, right-click Data Neighborhood and select Create New.

Select NI-DAQmx Global Virtual Channel and click Next.

Select Acquire Signals » Analog Input » Temperature » Thermocouple

Figure 2. Creating an NI-DAQmx Virtual Channel

Select ai0 or whichever physical channel you intend to connect your thermocouple. A physical channel is a terminal or pin at which you can measure or generate an analog or digital signal. A single physical channel can include more than one terminal or pin, as in the case of a differential thermocouple input channel. In this case, ai0 corresponds to TC0+ and TC0- on the NI-9211 pinout diagram.

Figure 3. Device Physical Channels

Click Next and enter a name for the global virtual channel or leave the default.

Click Finish to see the following screen in MAX:

Figure 4. Setting Up a Thermocouple Channel in MAX

On the settings tab, type in the minimum and maximum temperature values you expect to read from your thermocouple (0°C to 100°C by default).

Select your thermocouple type and CJC Source and CJC Value.

Connecting a Thermocouple to Your Device

The next step is to physically connect the thermocouple to your DAQ device.

Click the Connection Diagram tab in MAX to continue.

Figure 5. Thermocouple Connection Diagram

Each thermocouple wire has a positive lead and a negative lead. The connection diagram indicates which pins on your DAQ device should be wired according to the physical channel you selected. Connect the positive lead of the thermocouple to the TC+ terminal and the negative lead of the thermocouple to the TC– terminal. If you are unsure which of the thermocouple leads is positive and which is negative, check the thermocouple documentation or the thermocouple wire spool.

If you are using a shielded thermocouple, connect the COM terminal of your device to the shield and the shield to a common-mode voltage reference of the thermocouple. A common-mode voltage reference is a voltage that is within ±1.2 V of the common-mode voltage of the thermocouple. If you are using a floating thermocouple or a thermocouple within ±1.2 V of earth ground, connect COM and the shield to earth ground. The shield grounding methodology can vary depending on the application. Refer to Figure 6 for an illustration of a typical shielding configuration.

Testing the Signal

With MAX still open, click back on the NI-DAQmx Global Channel tab and click on the Run button. The temperature value of your thermocouple is displayed at the top of the screen.

Figure 7. Previewing a Thermocouple Measurement in MAX

You can choose to view the signal in tabular form or as a graph by selecting Graph from the Display Type dropdown. You also have the option of saving your NI-DAQmx Global Virtual Channel should you wish to refer to this configuration screen again in the future.